Molding materials containing epoxy resin are commonly put into wide use for the encapsulating of electronic devices such as transistors and ICs (integrated circuits). This is because the epoxy resin is well balanced in various properties such as electrical properties, moisture resistance, thermal resistance, mechanical properties and adhesion to inserts. In particular, a combination of o-cresol novolak epoxy resin with phenol novolak curing agent is very well balanced in these properties, and hence widely used as a base resin for IC-encapsulating molding materials.
In recent years, as electronic devices have come to be packaged in a high density, conventional packages of a pin-insertion type have shifted to packages of a surface-mounting type, and the latter is prevailing. In surface-mounting ICs, packages show a tendency to be made thin and compact so that packaging density can be made higher and packaging height can be made smaller. Accordingly, encapsulating mediums have come to be applied in a very small thickness because the volume held by device elements in a package must be made relatively larger.
Such surface-mounting packages differ from the conventional pin-insertion packages in the manner of packaging. In a packaging step for the pin-insertion packages, pins are inserted to a wiring board and thereafter these are soldered on the back of the wiring board. Hence, device elements are by no means directly exposed to high temperature. On the other hand, in a packaging step for the surface-mounting package, devices are provisionally fastened to the surface of a wiring board and then soldered by means of a solder bath or a reflowing assembly. Hence, the device elements are exposed to high temperature. As the result, in instances where a package has moistened, the absorbed moisture may expand abruptly at the time of soldering to crack the package. At present, this phenomenon is a serious problem in the fabrication of surface-mounting ICs.
In IC packages encapsulated with a usual base resin composition, the above problem is unavoidable. Hence, measures are taken such that ICs are moistureproof-wrapped to send them, or ICs are beforehand well dried to use them and then mounted on a wiring board. These measures, however, take much time and also require a high cost.
Accordingly, IC-encapsulating molding materials making use of a biphenyl skeleton type epoxy resin having a good moisture absorption and moisture resistance have been put into practical use for thin-type packages, because of its superior reflow crack resistance. This biphenyl skeleton type epoxy resin, however, has a problem that it has a low Tg (glass transition temperature), and there are limits within which this resin is usable.